The intestinal-enriched Kruppel-like factor (IKLF or KLF5) is a transcription factor that regulates intestinal epithelial cell proliferation. The central hypothesis of this application is that KLF5 is regulated by protein- protein interaction and post-translational modification. Preliminary studies indicate that KLF5 interacts with protein inhibitor of activated STAT1 (PIAS1), a SUMO (small ubiquitin-related modifier) ligase. PIAS1 increases KLF5 protein level in transfected cells and stimulates KLFS's transcriptional activity on cyclin D1 promoter. In addition, KLF5 and PIAS1 synergistically stimulate DNA synthesis. Thus, we hypothesize that (1) KLF5 is regulated by SUMOylation and this is promoted by PIAS1, and (2) PIAS1 positively regulates KLFS's transcriptional activity and enhances its ability to promote cell proliferation. Three specific aims are proposed: (1) to determine whether KLF5 is SUMOylated and whether this is facilitated by PIAS1, (2) to study whether PIAS1 positively regulates KLFS's transcriptional activity, and (3) to investigate whether PIAS1 enhances the pro-proliferative effect of KLF5. These aims will help elucidate the role of PIAS1 and SUMOylation in regulating KLFS's ability to control intestinal epithelial gene expression and proliferation. [unreadable] [unreadable] [unreadable]